From Spaceflight Now.

I enjoyed that article, thanks.

Quote;
Beside the large outflow channels, a variety of smaller 'dendritic' valley networks with a number of tributary valleys can be seen near the main channels. This indicates possible precipitation. End Quote

__________________
It's not what you know or don't know, but what you know that isn't so that will hurt you. Will Rogers
http://www.craterchains.com/ns/nspage.html

It means that there is evidence of smaller rivers and streams that point to the possibility of some form of precip - like rain or snow.

As Mars goes in and out of ice ages (caused by the changing axial tilt) and the ice caps melt and reform, over millenia some frost would form and melt. Perhaps that would sink in and over geologically long periods of time recharge the subsurface with ice that could then come to the surface periodically.

Just some thoughts...

As has been described as “catastrophic flooding”, and as Grizzly points to the words several other articles left off, “smaller rivers and streams that point to the possibility of some form of precipitation.” As stated in the article linked.

Aurora points out about the probable subsurface water / ice. It may come back to the surface via volcanic activities also. And, may do so some what violently. But that would not account for the massive hydraulics and flooding that is being noted.

A couple years ago when FieryIce and I started re-searching Mars pictures for crater chains we noted the vast amount of what appears to be catastrophic flooding having occurred there. Then last December she saw the APOD of “Mars Then and Now” and questioned me about the canals. That sent us into the re-searching and investigations of past astronomers notes, drawings, and pictures. It is well worth the investigation by any that are interested. Astronomers have given us the solar system, the stars and galaxies =D> Along with all the questions they pose.

__________________
It's not what you know or don't know, but what you know that isn't so that will hurt you. Will Rogers
http://www.craterchains.com/ns/nspage.html

Just as a matter of curiosity: is Percival Lowell one of those "past astronomers"?